Crystal structures of hydroxylamine and a 15-crown-5 hydroxylaminosolvate: first observation of a cyclic hydroxylamine dimer (NH₂OH)₂

Abstract

The crystal structure of neat hydroxylamine (I) has been determined for the first time using modern diffraction data, and the 15 crown 5 hydroxylaminosolvate (II) has been obtained. All hydrogen atoms were located from Fourier maps and refined anisotropically using non‑spherical form factors (NoSpherA2), unambiguously confirming the molecular NH₂OH form. The experimental N–O bond lengths, combined with a statistical analysis of CSD data using the 3σ rule, establish a straightforward geometric criterion: d(N–O) > 1.43 Å strictly distinguishes the molecular NH₂OH form from the zwitterionic NH₃⁺O⁻ form (d(N–O) ≤ 1.43 Å). Crucially, we demonstrate that the hydroxylammonium cation (NH₃OH⁺) also falls within this lower structural range, meaning that differentiation between the zwitterion and the cation requires analysis of the ionic environment rather than geometry alone. In II, two hydroxylamine molecules form the first experimentally observed cyclic (NH₂OH)₂ dimer via O–H···N hydrogen bonds. Periodic DFT calculations estimate the enthalpy of O–H···N and N–H···O bonds at ca. 30 and 20 kJ/mol, respectively. The term "hydroxylaminosolvate" is introduced for multicomponent crystals containing electroneutral hydroxylamine species, regardless of whether hydroxylamine is present as the molecular NH₂OH or the zwitterionic NH₃⁺O⁻ form. This study resolves the long-standing problem of distinguishing molecular NH₂OH from zwitterionic NH₃⁺O⁻ in multicomponent crystals of hydroxylamine.